1. Field of the Invention
The present invention relates to the technology of embedding message on a document image, and extracting the embedded message.
2. Description of the Related Art
In recent years, in digital image forming devices, such as a printer and a copying machine, the improvement in the quality of images is remarkable, and high-definition printed matter can now easily be obtained. That is, anyone can obtain printed matter demanded by image processing by a highly efficient scanner, printer, copying machine, and computer. Therefore, problems, such as an illegal copy of a document and falsification, have occurred. In order to prevent or inhibit such illegal copies or falsification, access control message has been embedded as watermark message in the printed matter itself in recent years. The following processes have been proposed as general realization method for such watermarks:                (1) embedding message by controlling the quantity of space between words;        (2) embedding message by rotating a character;        (3) embedding message by scaling of character; and        (4) embedding message by transforming a character.        
FIG. 1 shows the printed matter of the type which embeds message by controlling the quantity of space between words, e.g., the quantity of space between English words. Here, s and p are called a space. This space will be set to p1=(1+q) (p+s)/2, and s1=(1−q) (p+s)/2, if an embedding watermark message bit is “0”. It will be set to p1=(1−q) (p+s)/2, and s1=(1+q) (p+s)/2 if the embedding watermark message bit is “1”. The range of q is 0<q<1.
FIG. 2 illustrates a case in which watermark message is embedded by expanding or reducing a character size. For example, in cases where a character size is expanded rather than the original character, “1” is embedded (A in FIG. 2), and “0” is embedded in cases where the character size is reduced (B in FIG. 2). The character that is the embedded object may be a continuous character, a character of a prescribed interval, or a character of a prescribed position. In FIG. 2, since the character “m” is expanded and the character “u” is reduced, the watermark message “10” is embedded.
FIG. 3 is a figure illustrating a case in which watermark message is embedded by rotating a character (i.e., changing the lean of the character). For example, in cases where the character is rotated clockwise, “1” is embedded (C in FIG. 3), and “0” is embedded in cases where the character is rotated counterclockwise (D in FIG. 3). The character that is the embedded object may be a continuous character, a character of a prescribed interval, or a character of a prescribed position. In FIG. 3, since the character “m” is rotated clockwise and the character “t” is rotated counterclockwise, the message “10” is be embedded.
However, in cases where watermark message was embedded using the above-described conventional methods, a sense of incongruity is produced to the difference in a character size, the difference in the interval of a character, and the difference in the lean of a character.